1. Technical Field of the Invention
The present invention relates to a steam cooling type gas turbine which is adopted in a combined cycle power plant or the like, and more particularly to a sealing structure for sealing spaces between disks to prevent the leakage of cooling steam in the gas turbine.
2. Description of the Related Art
A combined cycle power plant is an electric power generating system in which a gas turbine plant and a steam turbine plant are combined, wherein the gas turbine is adapted to operate in a high temperature range of thermal energy and the steam turbine is employed in a low temperature range to recover and use thermal energy efficiently. This type of power generating system has been attracting attention in recent years.
In a combined cycle power plant such as mentioned above, the method of cooling the gas turbine of the topping cycle presents an important problem to be solved in the technical development of the combined cycle plant. As the result of trial-and-error attempts to realize a more effective cooling method there has been an evolution toward steam cooling systems, (also referred to as steam-jet cooling systems) in which steam obtained from the bottoming cycle is used as the coolant, and away from air-cooling systems in which compressed air is used as the coolant.
On the other hand, when the steam-jet cooling system is adopted, it is important to prevent the steam serving as the coolant from leaking along its path. To this end, many types of improvements in the sealing structure have been made.
A conventional sealing structure known heretofore will be described with reference to FIG. 9 and FIG. 10. The structure shown in these figures was first adopted in a gas turbine in which compressed air is employed as the coolant, and subsequently has been adopted in some steam cooling type gas turbines.
As is shown in FIG. 9, a rotor of a turbine section includes a plurality (ordinarily around four sets) of disks 1. In order to prevent a coolant 3 flowing through an inner space 2 of the rotor from flowing out to a gas path 4 of the turbine section while preventing a high-temperature gas 5 flowing through the gas path 4 of the turbine section from flowing into the inner space 2 of the rotor, annular projections (also referred to as disk lands) 6 are formed on the surfaces of adjacent disks 1 so as to face each other around a rotatable shaft, as shown in FIG. 10, wherein grooves 7 are formed in protruding end faces of the projections 6, respectively, so as to extend in a circumferential direction, and a seal plate (also referred to as a baffle plate) 8 divided into two or four parts in the circumferential direction in which the grooves 7 are disposed is inserted into the grooves 7. The baffle plate 8 is pressed against outer side walls of the grooves 7, respectively, by centrifugal force generated upon rotation of the turbine, whereby sealing is obtained.
With the conventional sealing structure described above, it is intended to realize the sealing by allowing the baffle plate to press against the outer side walls of the grooves formed in the arms of the disks under the action of the centrifugal force brought about by the rotation of the turbine. However, since the temperature differs from one disk to another, the elongation or stretch of the grooves in the radial direction will differ. Moreover, a difference can be observed among the disks with respect to the elongation in the radial direction under the influence of the centrifugal force.
On the other hand, since the baffle plate has a predetermined rigidity, a situation may arise where the baffle plate can not be pressed snugly and uniformly against the outer side walls of the grooves formed in the disks because of the difference in elongation, and as a result, minute gaps may be formed between the grooves and the baffle plates.
Consequently, the coolant confined within the interior of the rotor may flow to the gas path of the turbine section or the high-temperature gas may flow into the inner space from the gas path 4. Moreover, when the coolant continues to leak through the minute gaps, self-induced vibration of the baffle plate occurs causing abrasion of the baffle plate and other problems.
Thus, application of the sealing structure described above to the gas turbine where steam is used as the coolant, not to mention the case where compressed air is used as the coolant, will involve the loss of a large amount of steam from the bottoming cycle of an exhaust gas boiler or the like, causing a large degradation of the efficiency. Additionally, the amount of make-up steam will increase. For these reasons, the conventional sealing structure suffers serious problems concerning the validity of the system itself.